The present invention relates to methods and compositions for treating subterranean formations, and more specifically, to acid-generating fluid loss control additives that are useful in petroleum recovery operations including fracturing.
Providing effective fluid-loss control for well treatment fluids is highly desirable. A “treatment fluid” is a fluid used in a subterranean application for any purpose. The term “treatment” as used herein does not imply any particular action by the fluid or any component thereof. Fluid-loss control materials are often added to treatment fluids to aid in fluid loss control. These are additives that are generally designed to lower the volume of a filtrate that passes through a filter medium, e.g., permeable rock or a filter cake. Most attain their fluid-loss control from the presence of solvent-specific solids, or from polymers that rely on filter cake buildup and on viscoelasticity to inhibit flow into and through the formation. A variety of fluid-loss control materials have been used and evaluated, including foams, oil-soluble resins, acid-soluble particulates, graded salt slurries, linear viscoelastic polymers, and heavy metal-crosslinked polymers. Their respective comparative effects are well documented.
Another technique that has been developed to control fluid loss includes the addition of a hydrocarbon-based fluid (also referred to generically as an “oil”) to the treatment fluid in small droplets. The droplets are dispersed within the treatment fluid in an amount usually around about 1% to 5% by volume. Diesel is a common example. Other light hydrocarbons have been used as well. The droplets of the hydrocarbon-based fluid are thought to reduce the permeability of the polymer build-up in the filter cake. The oil droplets are thought to occupy space in the filter cake, but do not go into the rock to plug pore throats therein. The addition of approximately 2% to about 5% of an oil to a treatment fluid can lead to significant decreases in the amount of leak off of the treatment fluid. FIG. 1 illustrates fluid loss versus diesel oil concentration as an example. Although a reduction in fluid loss can be achieved using these methods, they also have many drawbacks. Oil droplets do not help clean up the filter cake, and may in fact, prevent or inhibit some breakers (such as oxidizers) from cleaning up the filter cake. Additionally, in a dry gas reservoir for example, the oil may become a third phase, which can damage the permeability of the formation to gas.